In light probing technology for inspecting a measurement target such as a semiconductor device, a measurement target is irradiated with light emitted from a light source and measurement light (reflected light) from the measurement target is detected by a light sensor to acquire a detection signal. In a magneto-optical frequency mapping (MOFM) method, which is a type of light probing technology, a magneto-optical crystal is arranged facing a measurement target and reflected light whose polarization state has changed in accordance with a magneto-optical effect of a magneto-optical crystal is detected by a light sensor. In this method, the presence/absence of abnormality of the measurement target is detected on the basis of a distribution of a magnetic field occurring in the measurement target. For example, in Patent Literature 1, a method of arranging a magneto-optical film with respect to a sample, acquiring an image of light obtained by reflecting linearly polarized light radiated to the magneto-optical film by a camera, and mapping a magnetic field of the sample and a flow of an electric current is disclosed.